The present invention relates generally to a technique for reproducing an information signal recorded on an optical disc along spiral or concentric tracks, and more particularly to a method for detecting a tracking error signal representing a deviation of a light spot on the disc with respect to the information track in a disc radial direction.
In an optical disc player, in order to reproduce a recorded information signal accurately, information tracks should be traced by a light spot precisely. For this purpose there have been proposed several methods for detecting the tracking error. One of the methods is called a wobbling method, in which the light spot on the disc is vibrated in the radial direction by a small distance in accordance with a position modulating signal having a given frequency and in which an amplitude modulated component contained in a reproduced RF signal due to the vibration is sampled by a sampling signal which is synchronised with the position modulating signal to obtain a tracking error signal. In this wobbling method, the light spot is moved periodically in the disc radial direction by a swingable mirror arranged in an optical path between a light source and an objective lens. For instance, the mirror is provided on electromechanical means such as an electrostrictive element to which is supplied the position modulating signal having the given frequency and the mirror is vibrated in accordance the modulation signal. In the known method the sampling pulse is fixed in phase with respect to the position modulating signal. In general, the sampling pulse is produced at a peak point of the amplitude modulated component. Therefore, if there is produced a drift or fluctuation of the electrostrictive element and its driving circuit due to temperature variation, secular variation, etc., there might be produced undesired phase difference between the sampling signal and the amplitude modulated component. That is to say, the sampling pulse is not always coincident with the peak point of the amplitude modulated component. Therefore, it is difficult to detect the tracking error precisely, so that the tracking control could not be effected accurately.